This invention relates generally to the field of rigid disc drives, and more particularly, but not by way of limitation, to a glide test head assembly for use in testing magnetic disc recording media surface characteristics.
Disc drives of the type known as xe2x80x9cWinchesterxe2x80x9d disc drives or hard disc drives are well known in the industry. Such disc drives record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent to the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator housing opposite to the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator housing rotates, the heads are moved radially across the data tracks along an arcuate path.
As the physical size of disc drives has decreased historically, the physical size of many of the disc drive components has also decreased to accommodate this size reduction. Similarly, the density of the data recorded on the magnetic media has been greatly increased. In order to accomplish this increase in data density, significant improvements in both the recording heads and recording media have been made.
For instance, the first rigid disc drives used in personal computers had a data capacity of only 10 megabytes, and were in the format commonly referred to in the industry as the xe2x80x9cfull height, 5xc2xcxe2x80x9d format. Disc drives of the current generation typically have a data capacity of over a gigabyte (and frequently several gigabytes) in a 3xc2xdxe2x80x3 package which is only one fourth the size of the full height, 5xc2xcxe2x80x3 format or less. Even smaller standard physical disc drive package formats, such as 2xc2xdxe2x80x3 and 1.8xe2x80x3, have been established. In order for these smaller envelope standards to gain market acceptance, even greater recording densities must be achieved.
The recording heads used in disc drives have evolved from monolithic inductive heads to composite inductive heads (without and with metal-in-gap technology) to thin-film heads fabricated using semi-conductor deposition techniques to the current generation of thin-film heads incorporating inductive write and magneto-resistive (MR) read elements. This technology path was necessitated by the need to continuously reduce he size of the gap in the head used to record and recover data, since such a gap size reduction was needed to reduce the size of the individual bit domain and allow greater recording density.
Since the reduction in gap size also meant that the head had to be closer to the recording medium, the quest for increased data density also lead to a parallel evolution in the technology of the recording medium. The earliest Winchester disc drives included discs coated with xe2x80x9cparticulatexe2x80x9d recording layers. That is, small particles of ferrous oxide were suspended in a non-magnetic adhesive and applied to the disc substrate. With such discs, the size of the magnetic domain required to record a flux transition was clearly limited by the average size of the oxide particles and how closely these oxide particles were spaced within the adhesive matrix. The smoothness and flatness of the disc surface was also similarly limited. However, since the size of contemporary head gaps allowed data recording and retrieval with a head flying height of twelve microinches (0.000012 inches, 12xcexcxe2x80x3) or greater, the surface characteristics of the discs were adequate for the times.
Disc drives of the current generation incorporate heads that fly at nominal heights of only about 2.0xcexcxe2x80x3, and products currently under development will reduce this flying height to 1.5xcexcxe2x80x3 or less. Obviously, with nominal flying heights in this range, the surface characteristics of the disc medium must be much more closely controlled than was the case only a short time ago.
In current disc drive manufacturing environments, it is common to subject each disc to component level testing before it is assembled into a disc drive. One type of disc test is referred to as a xe2x80x9cglidexe2x80x9d test, which is used as a go/no-go test for surface defects or asperities, or excessive surface roughness. A glide test typically employs precision spin stand and a p2t-glide test head. The glide test is performed with the test head flown at approximately half the flying height at which the operational read/write head will fly in the finished disc drive product. For instance, if the disc being glide tested is intended for inclusion in a disc drive in which the operational heads will fly at 2.0xcexcxe2x80x3, the glide test will typically be performed with the glide test head flying at 1.0xcexcxe2x80x3. During the glide test, any contact between the glide test head assembly and an irregularity on the surface of the disc being glide tested causes mechanical excitation of the slider body of the glide test head assembly, which in turn excites the piezo element carried on the glide test head assembly, causing it to output an electrical signal whose amplitude is reflective of the severity of the impact. The test system in which the glide test head assembly is used typically monitors this electrical signal, and compares its amplitude with a preset threshold level. If the output of the glide test head assembly exceeds the preset threshold level, the disc is considered unusable, and scrapped. If the glide test is completed without contact between the glide test head assembly and any surface defects, then the disc is passed for incorporation in a disc drive assembly on the assumption that there will be no contact between the operational heads and the discs during normal operation with an operational read/write head flying height twice that of the glide test head flying height.
Typical glide test head assemblies of the prior art mount the piezo element on a wing which extends laterally from the main slider body which includes the air bearing surfaces used to fly the glide test head assembly above the disc being tested. It is common to mount the piezo element at the extreme outer lateral edge of the laterally-extending wing, and to position the piezo element in a manner which causes it to overhang the trailing edge of the laterally-extending wing.
The piezo element is typically in the form of a rectilinear solid, and is mounted with its major axis extending in the direction of disc motion beneath the glide test head assembly, and its minor axis parallel with the plane of the disc surface.
The lead connections that carry the output signal from the piezo element are typically attached to the top and bottom surfaces of the piezo element in that portion of the piezo element which overhangs the trailing edge of the laterally-extending wing.
A consequence of this mounting location and orientation of the piezo element is that the natural resonant frequency of the piezo element is lowered. With the resonant frequency of the piezo element thus lowered, it has been found that the piezo element can be excited by air flow disturbances around the glide test head assembly, resulting in excitation of the piezo elementxe2x80x94and electrical signal output from the piezo elementxe2x80x94that is not caused by actual contact between the glide test head assembly and the disc under test.
Since typical test systems for glide testing cannot readily distinguish between piezo element outputs that are the result of true head/disc contact and those piezo element outputs that are merely the result of excitation of the piezo element by air flow disturbances, any detected excitation of the piezo element must be considered to be the result of contact between the glide test head assembly and the disc, and, in consequence, the disc may be found defective and scrapped, even though it has no actual surface defects that would cause operational problems in a disc drive. Such unnecessary scrapping of usable discs drives up the unit cost of disc drive products.
A need exists, therefore, for a glide test head assembly which includes a piezo element that has a high enough resonant frequency to be capable of distinguishing between spurious excitation caused by air flow disturbances and true head/disc contacts.
The present invention is a glide test head assembly in which the mounting orientation of the piezo element has been modified. The glide test head assembly mounts the piezo element with one of its major surfaces coincident with the surface of the laterally-extending wing on which it is mounted. The mounting orientation of the piezo element allows the piezo element to be mounted fully on the laterally-extending wing of the glide test head assembly, raising the resonant frequency of the piezo element, decreasing the glide test head assembly""s sensitivity to excitation caused by air flow disturbances and increasing the reliability of glide test results.
The manner in which the present invention is implemented, as well as other features, benefits and advantages of the invention, can best be understood by a review of the following Detailed Description of the Invention, when read in conjunction with an examination of the accompanying drawings.